I. Field of the Invention
The present invention relates to nonlinear devices and more particularly to a method for determining the affects of nonlinear device characteristics on the transfer of signals in a communications system. The invention further relates to a method of using the ratio of power in active versus inactive system channels in an orthogonal CDMA communication system, and more specifically using a Walsh Power Ratio to control the operation of nonlinear stages such as power amplifiers.
II. Related Art
One type of multiple access communication system used for transferring information among a large number of system users is based on code division multiple access (CDMA) spread spectrum techniques. Such communication systems are disclosed in the teachings of U.S. Pat. No. 4,901,307, which issued Feb. 13, 1990 under the title "Spread Spectrum Multiple Access Communication System Using Satellite Or Terrestrial Repeaters", and U.S. Pat. No. 5,691,974, which issued Nov. 25, 1997, under the title "Method And Apparatus For Using Full Spectrum Transmitted Power In A Spread Spectrum Communication System For Tracking Individual Recipient Phase Time And Energy," which are both assigned to the assignee of the present invention, and incorporated herein by reference.
These patents disclose communication systems in which generally mobile or remote system users or subscribers use transceivers to communicate with other system users or desired signal recipients, such as through a connected public telephone switching network. The transceivers communicate signals through gateways and satellites, or terrestrial base stations (also referred to as cell-sites or cells) using CDMA spread spectrum communication signals.
In a typical spread-spectrum communication system, one or more sets or pairs of preselected pseudorandom noise (PN) code sequences are used to modulate or `spread` user information signals over a predetermined spectral band prior to modulation onto a carrier for transmission as communication signals. PN spreading is a method of spread-spectrum transmission that is well known in the art, and produces a communication signal with a bandwidth much greater than that of the underlying data signal. In the base station- or gateway-to-user communication link, also referred to as the forward link, PN spreading codes or binary sequences are used to discriminate between signals transmitted by different base stations or between signals of different beams, satellites, or gateways, as well as between multipath signals. In the user terminal-to-base-station or -gateway communication link, also referred to as the reverse link, PN spreading codes or binary sequences are used to discriminate between signals intended for different beams, satellites, or gateways, as well as between multipath signals.
These codes are typically shared by all communication signals within a given cell or beam, and time shifted or offset between adjacent beams or cells to create different spreading codes. The time offsets provide unique beam identifiers which are useful for beam-to-beam handoff and for determining signal timing relative to basic communication system timing.
In a typical CDMA spread-spectrum communication system, channelizing codes are used to discriminate between signals intended for different users within a cell or between user signals transmitted within a satellite beam, or sub-beam, on a forward link. That is, each user transceiver has its own orthogonal channel provided on the forward link by using a unique `covering` or `channelizing` orthogonal code. Walsh functions are generally used to implement the channelizing codes, with a typical length being on the order of 64 code chips for terrestrial systems and 128 code chips for satellite systems. In this arrangement, each Walsh function of 64 or 128 chips is typically referred to as a Walsh symbol. The derivation of Walsh codes is more fully disclosed in U.S. Pat. No. 5,103,459 entitled "System And Method For Generating Signal Waveforms In A CDMA Cellular Telephone System", which is assigned to the assignee of the present invention and incorporated herein by reference.
The gateways or base stations, and satellites used in the systems discussed above use high power amplifiers (HPAs) to increase the power of signals being transferred to and from the satellites, gateways, and user terminals in or serviced by the communication system. It is desirable to achieve a significant amount of power increase in the signals, but at the same time waste as little power as possible in so doing. That is, expend power to amplify a signal but no more than necessary to achieve a desirable communication link. This relates to the desire to conserve power and, therefore, energy resources needed to power the amplifiers; and to minimize signal power to decrease mutual signal interference and increase system capacity.
It is also important to recognize that in a satellite communication system, the amount of power available for signal transmission is limited by the power-generating capacity of the satellite. To optimize the use of this power, it must be allocated between traffic signals, those intended to transfer information to and from users, and pilot signals, those intended to act as phase and time references, carefully. If too little power is allocated to the pilot signal, the user terminals cannot accumulate sufficient energy to synchronize their receivers with a gateway or base station. Conversely, if too much pilot signal power is transmitted, the amount of power available for traffic signals, and, thus, the number of users that can be supported, is reduced.
Therefore, to maximize the user capacity that can be handled by a satellite, the amount of pilot signal power transmitted must be accurately controlled. In addition, there are other shared resources such as paging and synchronization signals used to transfer system information, which act as shared resources similar to pilot signals. Such signals also impact power consumption in satellite or other power-limited or power-controlled communication systems. It is also desirable to minimize the amount of energy present in these signals to decrease mutual interference, in order to increase system capacity.
Power amplifiers in communication systems that operate at high intermodulation levels, such as those discussed above, generally operate close to their saturation point. The saturation point is the point at which the output power of the amplifier is no longer increasing with increasing input power. That is, after the saturation point has been reached, the output power of the power amplifier is substantially constant regardless of the input. Thus, the power amplifier exhibits a nonlinearity in its operation near the saturation point. The saturation region is also referred to as the gain compression region.
Intermodulation is a term that is used to describe the nonlinearity. For example, when a nonlinear device operates on a signal having multiple spectral components to produce an output signal, the output signal is comprised of spectral components that were not present in the original input signal. Some of the components can be removed by filtering, and do not cause significant distortion. Other components, however, cannot be removed by filtering. The components that cannot be removed by filtering give rise to nonlinear distortion. These components are commonly referred to as intermodulation products.
This intermodulation causes undesirable distortion in most communication systems. For example, in a CMDA communication system a CDMA signal is amplified prior to transmission over a communication channel. A nonlinear power amplifier is commonly used to provide this amplification. CDMA signals transmitted in real communication systems often exhibit a non-constant envelope which can result from a plurality of CDMA signals being multiplexed together to form a single multiplexed CDMA signal. Such a signal can result from several CDMA signals being combined onto a single carrier to form a CDMA channel, or several CDMA channels at different frequencies being combined into a signal for transmission. In any case, the multiplexed CDMA signal exhibits a non-constant envelope. Other well known causes can also give rise to the non-constant envelope phenomenon. As a result, the input power to the nonlinear amplifier traverses the input power range of the amplifier. Because, the nonlinear amplifier is nonlinear across its input range, the output signal exhibits undesirable nonlinear effects, such as intermodulation products.
Nonlinear distortion, such as that caused by intermodulation, is an undesirable effect which can destroy the information content of a signal in a communication system. Unfortunately, nonlinear distortion can also affect CDMA communication signal waveforms, such as those following the IS-95 standard, in such a manner that the channels no longer remain orthogonal. In essence, the nonlinear response causes the coded channels to "leak" or "bleed" into each other.
Traditionally, the performance of power amplifiers and other non-linear elements used to generate and amplify communication signals is quantified with two-tone, multi-tone, and noise loaded tests. In particular, the noise loaded test is referred to as the Noise Power Ratio, so called NPR, test, and measures how much energy density leaks into a narrow notch or the noise injected into the non-linear device under test.
However, there are some key differences between the intermodulation performance of noise and direct sequence spread spectrum signals (DS-SS). In particular, when the spread spectrum data modulation is what is referred to as one-dimensional, for example BPSK type, the envelope statistics of the DS-SS waveform are different than noise. Even where many information signals are multiplexed together such as found in CDMA communication systems (for CDM or CDMA) the DS-SS waveform has significantly different envelope statistics than band-pass noise if these signals share the same carrier frequency and carrier phase.
Band-pass noise has a power probability density function (PDF) that is chi-square with two degrees of freedom. Forward link CDMA channels or signals having many users or user signals (traffic channels) have an approximate power PDF of chi-square with one degree of freedom. Forward link CDMA type signals that conform to the IS-95 standard are a special case of this, where the CDM or CDMA signals are kept orthogonal with Walsh codes. This form of coding might be referred to as orthogonal CDMA, or O-CDMA for short, but, this is still BPSK modulation. Significant amounts of intermodulations on an IS-95 waveform mean that the channels are no longer orthogonal, they "leak" or "bleed" into each other.
The result of this leaking is that simple noise measurements on a channel do not reflect a true response or measure of communication system performance in a CDMA environment. This means that a Noise Power Ratio (NPR) test cannot simply be used to measure or determine the effects of power adjustments or the appropriate level of power to use with a particular amplification stage in a spread spectrum system. This is true because noise in the system tends to shift energy into other otherwise orthogonal channels.
Another issue where noise performance does not necessarily reflect CDMA performance is in separating what are referred to as the AM/AM and AM/PM effects. Both of these are well known to affect traditional noise measurement techniques such as NPR. However coherent BPSK demodulation is much more sensitive to AM/AM than AM/PM. To illustrate this point, the output power and phase characteristics of a conventional nonlinear power amplifier are illustrated in FIG. 1. In FIG. 1, a curve 102 illustrates the phase of the output versus the phase of an input sine wave. Such a curve is commonly referred to as an "AM-PM" plot. A curve 104 illustrates the magnitude of the output power versus the input power for a sinusoidal input. Such a curve is commonly referred to as an "AM-AM" plot. Curve 102 illustrates that the phase of the output power versus input power is non-constant over most of the operating region of a conventional nonlinear power amplifier. Similarly, curve 104 illustrates that the magnitude of the power output is nonlinear near a saturation region 106. In the case of the power amplifier illustrated in FIG. 1, saturation region 106 starts at about -4 dBm. It would be apparent to any person skilled in the art that the saturation region can extend over a different range of values.
Another issue besides simply testing the response or otherwise characterizing a communication system element such as a power amplifier, is the selection of power levels during operation. In this situation, it would be convenient to have a more accurate measure of performance that could be used once a system is deployed or in use, to at least occasionally characterize its operation and make adjustments to the operation of nonlinear devices.